This invention relates to magnetic elements, particularly but not exclusively to methods of distinguishing between magnetic elements and methods and apparats for reading magnetic data tags which include one or more magnetic elements, each of which can differ in coercivity, saturated dipole moment (i.e. response amplitude), orientation and bias field.
Co-pending PCT publication number WO99/35610 describes tags and reader systems primarily intended for tags fabricated from magnetic material of low coercivity, with elements at different orientations, in which data is recorded primarily by means of the orientation of the elements with respect to each other. The described system assumes that the coercivities of the tag elements are all the same, and arc very small compared to the interrogation field.
According to the invention, there is provided a method of reading a magnetic tag having at least one magnetic element, comprising interrogating the tag with a scanning magnetic field, determining transition data associated with changes in the magnetisation state of the at least one magnetic element, associating the transition data with one or more respective elements; and for each element, determining the element direction which corresponds to the transition data for that element.
Preferably, the element direction is determined by selecting the direction that minimises the scatter of the transition field vectors resolved along the direction of the element.
The transition data for each element can be grouped into two sets, which can be referred to as forward and reverse transitions. All those in the forward transition group have a positive component of the field vector dH/dt along the element vector, and all those in the reverse group have a negative component of dH/dt along the element vector. Mean field values, resolved along the element vector, can be calculated. The coercivity of the element is then calculated as half the difference between the forward and reverse mean values, and the bias field along the element is calculated as the sum of the forward and reverse mean value
According to the invention, there is further provided a method of distinguishing between a plurality of magnetic elements, comprising the steps of applying a scanning magnetic field to the elements, determining the direction of each of the elements, for each of the elements, determining the components of the field in the direction of the element at which the element switches magnetisation states; and from said components, determining, for each of the elements, respective characteristics of the element.
The invention further provides a method of determining, for a magnetic element, any one or more of a plurality of characteristics comprising the coercivity of the element, the local magnetic field bias resolved in the direction of the element and the orientation of the element, comprising the steps of applying a varying magnetic field to the element, determining the direction of the element, determining tie components of the field in the direction of the element at which the element switches magnetisation states; and from said components, determining the one or more characteristics of the element.
According to the invention, there is also provided a magnetic tag reader for reading a magnetic tag having at least one magnetic element, comprising means for interrogating the tag with a scanning magnetic field, means for determining transition data associated with changes in the magnetisation state of the at least one magnetic element, means for associating the transition data with one or more respective elements; and means for determining, for each element, the element direction which corresponds to the transition data for that element